This research continues a longitudinal study of mental and social development in a cohort of children observed repeatedly between 2 months and 4 years of age. The premise of this research is that learning in childhood occurs primarily within an interpersonal context; thus, a central goal is to describe how diverse infant and maternal behaviors relate to the ontogeny of central dimensions of children's mental and social competencies. Children participating in the assessment constitute a longitudinal cohort seen at 2, 5, 13, 20, and 48 months of age. Major objectives of this research are to study stability, continuity, and interaction in mental and social development over the first years of life. One notable aspect of the follow-up is a test of preschooler cognitive and emotional development that goes beyond IQ to assess a wide range of children's diverse interests and capabilities: These procedures assess natural science ability, bodily-kinesthetic skills, musical talents, distinctive styles of work, as well as linguistic and logical-mathematical capabilities. Because traditional psychometric measures of intelligence at preschool age typically sample from a narrow range of mental abilities, such measures are limited in terms of the information they provide about the possible relevance of antecedent variables, and they are also restricted in terms of the outcome variables they successfully predict. In contrast, the new assessment battery samples from a wide range of preschoolers' capabilities and interests and thus affords richer opportunities to respond to many theoretical and pragmatic questions that surround central issues of mental and social development.